Reabsorption of bile acids and lecithin has been demonstrated in rabbit and guinea pig gallbladders. The relative contribution of reabsorption to the failure of micellar solubilization of cholesterol and gallstone-forming bile is not known. We propose to extend our previous studies of bile acid reabsorption a) to the baboon, a gallstone susceptible species, and b) to the intrahepatic bile duct system on the theory that reabsorption of micellar constituents may also occur there. Ductal reabsorption is postulated to explain how bile supersaturated with cholesterol routinely appears after an overnight fast. So we will allow the hepatic secretory pressure to distend both the baboon gallbladder and ducts and measure the permeability before and after. Since we found alternate day feeding of a lithogenic diet in mice enhanced gallstone formation, we have begun studies on the effect of fasting on the lithogenicity of both human and mouse bile.